Conventionally, a method has been known in which the hydrocarbons in the exhaust gas are compulsorily enriched, and are brought into contact with a transition metal carrying metallo-silicate catalyst, such as a copper/zeolite catalyst, or a transition metal carrying oxide catalyst, such as a copper/alumina catalyst, to reduce and eliminate NO.sub.x in the exhaust gas. In the conventional methods, ethylene, propylene, or diesel fuel, such as kerosene and light oil, is used as the reducing agent hydrocarbon. However, in order to eliminate NO.sub.x, a large amount of the reducing agent is necessary. As a concrete example, under practical conditions of diesel exhaust gas, in order to reduce and eliminate NO.sub.x, a hydrocarbon weight of two and half times to four times as much as that of the NO.sub.x, is necessary.
Contrary to the above, in Japanese Patent Application Laid-open No. 6-327974, it is described that the required amount of reducing agent can be substantially reduced by using an alcohol (especially ethanol) as the reducing agent. However, ethanol and propanol are more expensive as compared to ethylene and diesel fuel; and moreover it is necessary to separately prepare an additional substance as the reducing agent, so that the above art cannot be regarded as practical.
In Japanese Patent Application Laid-open No. 5-133218, it is proposed that fuel be reformed to a plurality of substances, including an alcohol which can be used as the reducing agent. However, since the reducing agent which is obtained after reforming is a kind which does not aim at an oxygen-containing compound such as, especially, an alcohol, there are disadvantages in that:
i) the yield of an oxygen-containing compound such as an alcohol is less, and PA0 ii) caulking carbon is deposited on the surface of the reforming catalyst, so that the performance of the reforming catalyst is easily lowered.